cases, the color and Z values are maintained for each sample and performance should
be the same.  For long, thin polygons, area sampling helps to avoid breakup.

Coverage Masks

If the database being rendered can be sorted front-to-back, a coverage mask can be
used to get 16 sample points, each of which is represented by a bit in a 16-bit word.
Thus, one 16-bit word per pixel provides all the necessary coverage information (and
will help with transparency computations as well).

Sample Memory

The number of anti-aliasing samples available depends upon the amount of memory
applied to the problem when performing point or area sampling.
 

5.6.8 Texture Mapping
 
RealityEngine has been designed from the start to be able to display complex, texture
mapped scenes at high (30-60Hz) frame rates.  While maintaining compatibility with
the earlier PowerVision and SkyWriter systems was one design goal, dramatically
improved performance and image quality was of paramount importance.

Textures may be derived from actual photographs or generated synthetically.  They
may contain full or partial transparency at any point and support up to 36-bit RGB
true color.  The following sections detail the various methodologies used to achieve
high-quality, realistic imagery.

Currently, the highest quality texture mapping in use in the real-time image
generation industry is based upon a technique called Trilinear MIPmapping.  This
technique has been available on Silicon Graphics IRIS 4D/VGXT series of graphics
supercomputers as the highest level of texture quality, with the next lower mode,
Bilinear MIPmapping, having the best performance.  The RealityEngine graphics
subsystem has been designed from the start as a texture mapping machine with
Trllliiear MIPmapping as the basic level of texture quality.

Texture Components

Not every texture requires full color.  Some textures need only vary the existing color
or transparency of a surface, as in partially transparent clouds or water surfaces.
RealityEngine supports textures of the different types, as detailed below.